The cities, gardens, cliffs and isles show that the Earth is:

The poem mentions 'cities and gardens, and cliffs and isles, And people upon you for thousands of miles?' This list of diverse and vast geographical features spanning thousands of miles demonstrates the enormous size of the Earth. The poet also exclaims 'You are so great and I am so small' confirming the contrast between the huge Earth and the small individual. Hence option (1), huge, is correct.